Grate a few uncooked potatoes, add them to the boiling stock and simmer for ½ an hour. Rub through a sieve and put the soup back in a saucepan. A few minutes before serving, add the watercress, finely chopped. Put the yolks of 4 eggs in the soup tureen, mix them thoroughly with a cup of warm milk and 1 tablespoon of butter, adding gradually a few tablespoons of the hot soup. Finally pour the soup into the tureen over the egg mixture. Season with salt and pepper and sprinkle with grated cheese, according to taste.